The image of Slide 4 which lists the recommended materials for the "Tech Suit" to me shows not the outer armor, but focuses more on the inner ribbed or padded section of the thigh armor, which we have always accepted as either rigged or flexible or attached to the undersuit and not the outer hard armor.
Unless you referring to slides 12-18 being labeled as "Combat Suit?" Those same slides also have other items like the the Shins, Forearm, and boots present. As such, I feel that is simply more along the lines of those are 343i's wording for the break down, or models. Like, all things show on those slides were on the "Combat Suit" in game model, and the other items were model separately and attached to/separate layers or models, in game. Otherwise, by your reasoning, there is also no hard armor on the shin or forearms. Since additionally the thigh area bears the weight of additional items like grenades, ammo pouches, buckles, and clips, it would seem they were intended to be at least some form of rigid. Slide 48 for the thigh pouches even does state "The thigh packs are directly attached to the armor," and combined with slide 48, seems to indicate at least the middle of the outer thigh is considered "Armor" and as such rigid.
However, at the end of the day, the idea has always been to replicate the appearance or the look of the in game costume, not the exact materials used, or in this case now that know, intended to be used. We will continue to accept hard armor outer thighs for Tier 3, and will now consider more flexible material for all but the outer thigh "mounting plate" explicitly referred to as "Armor" in the guide, as long as the costume looks like it "Stepped out of the game."
If anything this guide will just raise the bar on other visible areas of the under suit/tech suit. (We can only grade what we can see, and if a part is covered by another piece we don't expect it to there, like SPARATNs who only have the Tech Suit details on the exposed areas, we don't look for it, but if it is there, that is a definite plus.)